1. Field of the Invention
The present invention relates generally to wireless communication systems and more specifically, to transmission power control of sounding reference signals.
2. Description of the Art
A communication system includes a DownLink (DL) that conveys signals from at least one Transmission Point (TP) to User Equipments (UEs), and an UpLink (UL) that conveys signals from UEs to at least one Reception Point (RP). A UE, also referred to as a fixed or mobile terminal or a mobile station, includes a wireless device, a cellular phone, a personal computer device, and the like. A TP or an RP is generally a fixed station and is also referred to as a Base Transceiver System (BTS), a NodeB, an access point, and the like.
A communication system also supports several signal types of transmissions including data signals conveying information content, control signals enabling proper processing of data signals, and Reference Signals (RS), also known as pilots, enabling coherent demodulation of data or control signals or providing Channel State Information (CSI) corresponding to an estimate of a channel medium experienced by their transmissions.
DL data information is conveyed through a Physical DL Shared CHannel (PDSCH). DL Control Information (DCI) includes Scheduling Assignments (SAs) for Physical UL Shared CHannel (PUSCH) transmissions from UEs (UL SAs) or for PDSCH receptions by UEs (DL SAs). The SAs are conveyed through DCI formats transmitted in respective Physical DL Control CHannels (PDCCHs). In addition to SAs, PDCCHs may convey DCI that is common to all UEs or to a group of UEs. DCI also includes ACKnowledgment (ACK) information associated with a Hybrid Automatic Repeat reQuest (HARQ) ACK (HARQ-ACK) process transmitted to UEs from at least one TP through Physical HARQ-ACK Indicator CHannels (PHICHs) in response to respective receptions at RPs of data Transport Blocks (TBs) transmitted from the UEs.
UL data information is conveyed through a Physical UL Shared CHannel (PUSCH). UL Control Information (UCI) is conveyed through a Physical UL Control CHannel (PUCCH), unless a UE also transmits a PUSCH, in which case the UE may convey at least some UCI in the PUSCH. UCI includes HARQ-ACK information and is transmitted in response to a reception by a UE of data TBs. HARQ-ACK signaling is periodic or dynamic, if a respective reception of data TBs by a UE is semi-persistently (periodically) scheduled without a respective PDCCH or dynamically scheduled by a PDCCH. Other periodically transmitted UCI signaling includes DL CSI informing a NodeB of a channel medium experienced by a signal transmission to a UE and Scheduling Request (SR) informing a NodeB that a respective UE has data to transmit. A UL RS is used for demodulation of data or control signals, in which case the UL RS is referred to as DeModulation RS (DMRS), or to sound a UL channel medium and provide NodeBs with UL CSI, in which case it is referred to as a Sounding RS (SRS).
Typically, PDCCHs are a major part of a DL overhead. One method for reducing this overhead is to scale its size according to the resources required to transmit the PDCCHs and PHICHs in a DL Transmission Time Interval (TTI). Assuming Orthogonal Frequency Division Multiple Access (OFDMA) as the DL transmission method, a Control Format Indicator (CFI) parameter is transmitted through a Physical Control Format Indicator CHannel (PCFICH) to indicate a number of OFDM symbols allocated to a DL control region during a DL TTI.
FIG. 1 is a diagram illustrating a conventional structure for a DL control region in a DL TTI.
Referring to FIG. 1, a DL TTI includes one subframe having M symbols and a DL control region occupies a first N subframe symbols 110. The remaining M-N subframe symbols are primarily used to transmit PDSCHs 120. A PCFICH 130 is transmitted in some sub-carriers, also referred to as Resource Elements (REs) of the first symbol and conveys 2 bits indicating a PDCCH size of M=1, or M=2, or M=3 symbols. A PHICH 140 is also transmitted in some REs of the first symbol. Moreover, some symbols also contain RS REs, 150 and 160, that are common to all UEs for each of the TP antenna ports which in FIG. 1 are assumed to be two ports. The main purposes of UE-Common RS (CRS) are to enable a UE to obtain a channel estimate for its DL channel medium and to perform other measurements and functions. The remaining REs in the DL control region are used to transmit PDCCHs.
PDCCHs conveying SAs are not transmitted at predetermined locations in a DL control region and, as a consequence, each UE needs to perform multiple decoding operations to determine whether it has an SA in a DL subframe. To assist a UE with the multiple decoding operations, REs carrying a PDCCH are grouped into Control Channel Elements (CCEs) in the logical domain. For a given number of DCI format bits, a number of PDCCH CCEs depends on a channel coding rate, assuming Quadrature Phase Shift Keying (QPSK) as the modulation scheme. For UEs experiencing low or high DL Signal-to-Interference and Noise Ratio (SINR), TPs may respectively use a low or high channel coding rate for a PDCCH transmission in order to achieve a desired BLock Error Rate (BLER). Therefore, a PDCCH transmission to a UE experiencing low DL SINR may require more CCEs than a PDCCH transmission to a UE experiencing high DL SINR (different power boosting of REs of a CCE may also apply). Typical CCE aggregation levels for a PDCCH are, for example, of 1, 2, 4, and 8 CCEs.
For a PDCCH decoding process a UE determines a search space for candidate PDCCHs according to a common set of CCEs for all UEs (Common Search Space or CSS) and according to a UE-dedicated set of CCEs (UE-Dedicated Search Space or UE-DSS). The CSS may consist of the first NCCEUE-CSS CCEs in the logical domain. The UE-DSS is determined according to a pseudo-random function having as inputs UE-common parameters, such as a subframe number or a total number of CCEs in a subframe, and UE-specific parameters such as a Radio Network Temporary Identifier (RNTI). For example, for CCE aggregation levels Lε{1,2,4,8}, the CCEs for PDCCH candidate m are given by L·{(Yk+m) mod └NCCE,k/L┘}+i where NCCE,k is a total number of CCEs in subframe k, i=0, . . . , L−1, m=0, . . . , M(L)−1, M(L) is a number of PDCCH candidates to monitor in a search space, and └ ┘ is the “floor” function rounding a number to its immediately smaller integer. Exemplary values of M(L) for Lε{1,2,4,8} are, respectively, {0, 0, 4, 2} in the UE-CSS, and {6, 6, 2, 2} in the UE-DSS. For the CSS, Yk=0. For the UE-DSS, Yk=(A·Yk-1)mod D where Y−1=RNTI≠0, A=39827 and D=65537.
PDCCHs conveying information to multiple UEs, such as a PDCCH conveying Transmission Power Control (TPC) commands for UEs for adjusting respective PUSCH or PUCCH transmission powers, are transmitted in the CSS. If enough CCEs remain in the CSS after transmitting PDCCHs conveying DCI to multiple UEs in a subframe, the CSS is also used to transmit PDCCHs providing SAs with specific DCI formats. The UE-DSS is exclusively used to transmit PDCCHs providing SAs. For example, the CSS may consist of 16 CCEs and support 2 PDCCHs with L=8 CCEs, or 4 PDCCHs with L=4 CCEs, or 1 PDCCH with L=8 CCEs and 2 PDCCHs with L=4 CCEs. The CCEs for the CSS are placed first in the logical domain (prior to an interleaving of CCEs).
FIG. 2 is a diagram illustrating a conventional PDCCH transmission process.
Referring to FIG. 2, after channel coding and rate matching, encoded bits of DCI formats are mapped to CCEs in the logical domain. The first 4 CCEs (L=4), CCE1 201, CCE2 202, CCE3 203, and CCE4 204 are used to transmit PDCCH to UE1. The next 2 CCEs (L=2), CCE5 211 and CCE6 212, are used to transmit PDCCH to UE2. The next 2 CCEs=2), CCE7 221 and CCE8 222, are used to transmit PDCCH to UE3. Finally, the last CCE (L=1), CCE9 231, is used to transmit PDCCH to UE4. The DCI format bits of a PDCCH is scrambled 240 with a binary scrambling code and are subsequently modulated 250. Each CCE is further divided into Resource Element Groups (REGs). For example, a CCE including 36 REs is divided into 9 REGs, each including 4 REs. Interleaving 260 is applied among REGs (blocks of 4 QPSK symbols). For example, a block interleaver is used with interleaving performed on symbol-quadruplets (4 QPSK symbols corresponding to 4 REs of a REG) instead of on individual bits. After REG interleaving, a resulting series of QPSK symbols is shifted by J symbols 270, and finally each QPSK symbol is mapped to an RE 280 in the DL control region of a subframe. Therefore, in addition to RS from TP antenna ports, 291 and 292, and other control channels such as a PCFICH or a PHICH 293, REs in a DL control contain QPSK symbols corresponding to DCI formats for UE1 294, UE2 295, UE3 296, and UE4 297.
After the reception of a PDSCH, a UE transmits HARQ-ACK information in a PUCCH to indicate the correct (ACK) or incorrect (NACK) reception of data TBs in a PDSCH.
FIG. 3 is a diagram illustrating a conventional structure for HARQ-ACK signal transmission in a PUCCH.
Referring to FIG. 3, HARQ-ACK signals, and RS enabling coherent demodulation of HARQ-ACK signals, are transmitted in one slot 310 of a PUCCH subframe including 2 slots. HARQ-ACK information bits 320 modulate 330 a Zadoff-Chu (ZC) sequence 340, for example using BPSK or QPSK, which is then transmitted after performing an Inverse Fast Fourier Transform (IFFT) operation. Each RS 350 is transmitted through an unmodulated ZC sequence.
For a UL system BandWidth (BW) including NRBmax,UL Resource Blocks (RBs), where each RB includes NscRB=12 REs, a ZC sequence ru,v(α)(n) is defined by a Cyclic Shift (CS) α of a base ZC sequence ru,v (n) according to ru,v(α)(n)=ejαnru,v(n), 0≦n<MscRS, where MscRS=mNscRB is a length of a ZC sequence, 1≦m≦NRBmax,UL, and ru,v(n)=xq (n mod NZCRS) where the qth root ZC sequence is defined by
                    x        q            ⁡              (        m        )              =          exp      (                                    -            j                    ⁢                                          ⁢          π          ⁢                                          ⁢                      qm            ⁡                          (                              m                +                1                            )                                                N          ZC          RS                    )        ,0≦m≦NZCRS−1 with q given by q=└q+½┘+v·(−1)└2q┘ and q given by q=NZCRS·(u+1)/31. The length NZCRS of a ZC sequence is given by the largest prime number such that NZCRS<MscRS. Multiple RS sequences are defined from a single base sequence through different values of α. A PUCCH transmission is assumed to be in one RB (MscRS=NscRB).
FIG. 4 is a diagram illustrating a conventional transmitter for a ZC sequence.
Referring to FIG. 4, a mapper 420 maps a ZC sequence 410 to REs of an assigned transmission BW as they are indicated by RE selection unit 425. Subsequently, an IFFT is performed by IFFT unit 430, a CS is applied to the output by CS unit 440, followed by scrambling with a cell-specific sequence using scrambler 450, a Cyclic Prefix (CP) is inserted by CP insertion unit 460, and the resulting signal is filtered by filter 470. Finally, a transmission power PPUCCH is applied by power amplifier 480 and the ZC sequence is transmitted 490.
Different CSs of a ZC sequence provide orthogonal ZC sequences. Therefore, different CSs of a same ZC sequence are allocated to different UEs in a same PUCCH RB and achieve orthogonal multiplexing for respective HARQ-ACK signal and RS transmissions. Orthogonal multiplexing can also be in the time domain using Orthogonal Covering Codes (OCCs) where PUCCH symbols used for HARQ-ACK signal or RS transmission in each slot are respectively multiplied with a first OCC and a second OCC. For example, for the structure in FIG. 3, HARQ-ACK signal transmission is modulated by a length-4 OCC, such as a Walsh-Hadamard (WH) OCC, while RS transmission is modulated by a length-3 OCC, such as a Discrete Fourier Transform (DFT) OCC. In this manner, the multiplexing capacity is increased by a factor of 3 (determined by the OCC with the smaller length Noc). The WH OCCs, {W0, W1, W2, W3}, and DFT OCCs, {D0, D1, D2}, are:
            [                                                  W              0                                                                          W              1                                                                          W              2                                                                          W              3                                          ]        =          [                                    1                                1                                1                                1                                                1                                              -              1                                            1                                              -              1                                                            1                                1                                              -              1                                                          -              1                                                            1                                              -              1                                                          -              1                                            1                              ]        ,            [                                                  D              0                                                                          D              1                                                                          D              2                                          ]        =                  [                                            1                                      1                                      1                                                          1                                                      e                                                      -                    j                                    ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                                      π                    /                    3                                                                                                      e                                                      -                    j                                    ⁢                                                                          ⁢                  4                  ⁢                                                                          ⁢                                      π                    /                    3                                                                                                          1                                                      e                                                      -                    j                                    ⁢                                                                          ⁢                  4                  ⁢                                                                          ⁢                                      π                    /                    3                                                                                                      e                                                      -                    j                                    ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                                      π                    /                    3                                                                                      ]            .      
Table 1 presents a mapping of a PUCCH resource nPUCCH to an OC noc and a CS α for a HARQ-ACK signal and RS transmission. For brevity, the RS associated with the HARQ-ACK signal will not be mentioned in the following. As a PUCCH is assumed to be transmitted over 1 RB including 12 REs, there is a total of 12 CS for a ZC sequence.
TABLE 1HARQ-ACK Signal and RS Resource Mapping to OC and CSOC noc for HARQ-ACK Signal and for RSCS αW0, D0W1, D1W3, D20nPUCCH = 0nPUCCH = 121nPUCCH = 62nPUCCH = 1nPUCCH = 133nPUCCH = 74nPUCCH = 2nPUCCH = 145nPUCCH = 86nPUCCH = 3nPUCCH = 157nPUCCH = 98nPUCCH = 4nPUCCH = 169nPUCCH = 1010nPUCCH = 5nPUCCH = 1711nPUCCH = 11
A UE can determine a conventional PUCCH resource nPUCCH for its HARQ-ACK signal transmission either through explicit signaling from serving TP(s) or through implicit signaling. The latter is based on CCEs used to transmit a PDCCH conveying a respective DL SA in response to which a UE transmits a HARQ-ACK signal. A one-to-one mapping may exist between conventional PUCCH resources used to transmit HARQ-ACK signals and CCEs used to transmit PDCCHs. For example, for UEs with one transmitter antenna port and a PDCCH transmission structure as in FIG. 2, a UE determines a conventional PUCCH resource for HARQ-ACK signaling from a CCE with a lowest index from a respective DL SA transmission. Then, UE1, UE2, UE3, and UE4 can respectively use PUCCH resource 1, 5, 7, and 9. If all PUCCH resources within an RB are used, resources in the immediately next RB are used. In general, a UE can determine a conventional PUCCH resource nPUCCH for HARQ-ACK signaling as nPUCCH=nCCE+NPUCCH where nCCE is a CCE with a lowest index for a respective DL SA and NPUCCH is TP-specific offset that is informed to UEs by higher layer signaling.
Improving coverage and cell-edge throughput are key objectives in communication systems. Coordinated Multi-Point transmission/reception (CoMP) is an important technique to achieve these objectives. CoMP operation relies on the fact that when a UE is in a cell-edge region, it is able to reliably receive a signal combined at a set of TPs (DL CoMP) and reliably transmit a signal combined at a set of RPs (UL CoMP). DL CoMP schemes can range from simple ones of interference avoidance, such as coordinated scheduling, to more complex ones requiring accurate and detailed channel information such as joint transmission from multiple TPs. UL CoMP schemes can also range from simple ones where PUSCH scheduling is performed considering a single RP to more complex ones where received signal characteristics and generated interference at multiple RPs are considered.
FIG. 5 is a diagram illustrating a conventional UL CoMP operation.
Referring to FIG. 5, a signal transmitted by a UE 510 is received from two RPs, RP1 520 and RP2 530. Scheduling coordination between the two RPs and combining of the respective received signals is facilitated by a fast backhaul link such as an optical fiber link.
Support of UL CoMP introduces new requirements for HARQ-ACK signaling in a PUCCH. As conventional HARQ-ACK signaling scrambles a respective ZC sequence with a respective RP-specific (cell-specific) sequence, it is not possible to support orthogonal multiplexing of HARQ-ACK signals in a same RB for reception at multiple RPs. For this reason, separate PUCCH RBs should be used for UL CoMP reception of HARQ-ACK signals. The scrambling of such HARQ-ACK signals is with a scrambling sequence that is common for all RPs constituting a set of UL CoMP RPs for a respective set of UEs (a CoMP-set specific ZC sequence is UE-specific and signaled to a UE by higher layer signaling).
The need to provide non-conventional PUCCH resources to support orthogonal multiplexing of HARQ-ACK signals for reception at multiple RPs, relative to conventional PUCCH resources supporting orthogonal multiplexing of HARQ-ACK signals for reception at a single RP, is associated with a respective increase in the UL overhead which reduces UL throughout.
Additionally, if both a conventional PDCCH and non-conventional PDCCH types are transmitted in a same subframe, collisions of PUCCH resources may occur, if the respective CCEs of the various PDCCH types are independently indexed and non-conventional PUCCH resources may then need to be configured for each of the non-conventional PDCCH types.
Therefore, there is a need to reduce an overhead resulting from assigning non-conventional PUCCH resources for transmissions of HARQ-ACK signals.
There is another need to provide mappings for compressing an amount of non-conventional PUCCH resources for HARQ-ACK signaling.
Finally, there is another need for indicating to a UE whether to use a conventional PUCCH resource or a non-conventional PUCCH resource for its HARQ-ACK signaling.